The general arrangement of a core shroud 2 inside a reactor pressure vessel (RPV) 4 is depicted in FIG. 1A. Feedwater is admitted into RPV 4 via a feedwater inlet (not shown) and a feedwater sparger 6, which is a ring-shaped pipe having suitable apertures for circumferentially distributing the feedwater inside the RPV. The feedwater from sparger 6 flows downwardly through the downcomer annulus 8, which is an annular region between core shroud 2 and RPV 4.
Core shroud 2 is a stainless steel cylinder surrounding the nuclear fuel core, the location of which is generally designated by numeral 10 in FIG. 1. The core is made up of a plurality of fuel bundle assemblies. Each array of fuel bundle assemblies is supported at the top by a top guide and at the bottom by a core plate (neither of which are shown). The core top guide provides lateral support for the top of the fuel assemblies and maintains the correct fuel channel spacing to permit control rod insertion.
The water flows through downcomer annulus 8, around the bottom edge of the shroud and into the core lower plenum 12. The water subsequently enters the fuel assemblies, wherein a boiling boundary layer is established. A mixture of water and steam enters core upper plenum 14 under the shroud head 16. The steam-water mixture then flows through vertical standpipes (not shown) atop the shroud head and enters steam separators (not shown), which separated liquid water from steam. The liquid water then mixes with feedwater in the mixing plenum, which mixture then returns to the core via the downcomer annulus. The steam is withdrawn from the RPV via a steam outlet.
The BWR also includes a coolant recirculation system which provides the forced convection flow through the core necessary to attain the required power density. A portion of the water is sucked from the lower end of the downcomer annulus 8 via recirculation water outlet (not visible in FIG. 1A) and forced by a centrifugal recirculation pump (not shown) into jet pump assemblies 18 (two of which are shown in FIG. 1A)) via recirculation water inlets 20. The BWR has two recirculation pumps, each of which provides the driving flow for a plurality of jet pump assemblies. The jet pump assemblies are circumferentially distributed around the core shroud 2.
The core shroud 2 (see FIG. 1B) comprises a shroud head flange 2a for supporting the shroud head 16; a circular cylindrical upper shroud wall 2b having a top end welded to shroud head flange 2a; an annular top guide support ring 2c welded to the bottom end of upper shroud wall 2b; a circular cylindrical middle shroud wall having a top end welded to top guide support ring 2c and consisting of upper and lower shell sections 2d and 2e joined by mid-shroud attachment weld W; and an annular core plate support ring 2f welded to the bottom end of the middle shroud wall and to the top end of a lower shroud wall 2g. (Some boiling water reactors have middle shroud wall comprising three stacked shell sections joined by attachment welds.) The entire shroud is supported by a shroud support 22, which is welded to the bottom of lower shroud wall 2g, and by annular jet pump support plate 24, which is welded at its inner diameter to shroud support 22 and at its outer diameter to RPV 4.
The material of the shroud and associated welds is austenitic stainless steel having reduced carbon content. The heat-affected zones of the shroud girth welds, including the mid-shroud attachment weld, have residual weld stresses. Therefore, the mechanisms are present for mid-shroud attachment weld W and other girth welds to be susceptible to intergranular stress corrosion cracking (IGSCC).
Stress corrosion cracking in the heat affected zone of any shroud girth seam weld diminishes the structural integrity of the shroud, which vertically and horizontally supports the core top guide and the shroud head. In particular, a cracked shroud increases the risks posed by a loss-of-coolant accident (LOCA) or seismic loads. During a LOCA, the loss of coolant from the reactor pressure vessel produces a loss of pressure above the shroud head and an increase in pressure inside the shroud, i.e., underneath the shroud head. The result is an increased lifting force on the shroud head and on the upper portions of the shroud to which the shroud head is bolted. If the core shroud has fully cracked girth welds, the lifting forces produced during a LOCA could cause the shroud to separate along the areas of cracking, producing undesirable leaking of reactor coolant. Also, if the shroud weld zones fail due to stress corrosion cracking, there is a risk of misalignment from seismic loads and damage to the core and the control rod components, which would adversely affect control rod insertion and safe shutdown.
Thus, the core shroud needs to be examined periodically to determine its structural integrity and the need for repair. Ultrasonic inspection is a known technique for detecting cracks in nuclear reactor components. The inspection area of primary interest is the outside surface of the cylindrical core shroud at the horizontal mid-shroud attachment welds. However, the core shroud is difficult to access. Installation access is limited to the annular space between the outside of the shroud and the inside of the reactor pressure vessel, between adjacent jet pumps. Scanning operation access is additionally restricted within the narrow space between the shroud and jet pumps, which is about 0.5 inch wide in some locations. The inspection areas are highly radioactive, and are located under water 50 to 65 feet below the operator's work platform. Thus, inspection of the core shroud in operational nuclear reactors requires a robotic scanning device which can be installed remotely and operated within a narrowly restricted space.